Cigarette smoke (CS) affects 1 billion people worldwide and causes 6 million premature deaths from associated diseases each year. Acute lung injury (ALI)/ acute respiratory distress syndrome (ARDS) is a life- threatening condition with 40% mortality. Emerging epidemiology studies have shown that CS predisposes lungs to infections and increases the likelihood of ALI/ ARDS. However, the mechanism of adverse effects of CS on ALI is unknown. There is an urgent need for strategies to prevent and treat ALI/ ARDS in smokers. The objectives of this application is to use human lung microvascular endothelial cells (LMVEC) and preclinical animal models to identify mechanisms by which CS impairs pulmonary endothelial barrier function and increases vulnerability to ALI after bacterial infection and to develop strategies to prevent and restore endothelial barrier function and ALI in susceptible populations. This study uses state-of-art technologies and comprehensive approaches to address a central hypothesis that CS increases endothelial cell permeability and predisposes lungs to ALI after infection by a mechanism in which oxidants from CS and mitochondria cause Akt oxidation and subsequent GSK3?-dependent activation of histone deacetylase 6 (HDAC6), this results in microtubule deacetylation, leading to translocation of dynamin-related protein (Drp)1 to mitochondria, culminating in mitochondrial fission and dysfunction and subsequent release of mitochondrial damage- associated molecular patterns (mtDAMPs). Aim 1: We will first determine the role of HDAC6 in CS priming for ALI after Pseudomonas aeruginosa infection by using global HDAC6 knockout mice, endothelium-specific lentiviral HDAC6 miRNA silencing in vivo, and LMVEC isolated from HDAC6 knockout mice; we will then determine the mechanism by which CS exposure leads to HDAC6 activation. Aim 2: We will determine the role of HDAC6 in CS-induced mitochondrial fission and dysfunction and their roles in mediating increased endothelial permeability and priming for ALI after P. aeruginosa infection. We will first characterize the effects of CS exposure on Drp1 translocation and on mitochondrial fission, fusion, function, and release of mtDNA in vitro and in vivo; we will then determine the effect of blocking HDAC6 or ?-tubulin deacetylation on the adverse effects of CS on mitochondria in vitro and in vivo; finally, we will determine the roles of mitochondrial fission, mitochondrial ROS and mtDAMPs in CS-induced increased endothelial permeability in vitro and CS priming for ALI in vivo. This study will provide innovative insights into the effect of CS on lung endothelial barrier function. Establishment of the links among HDAC6, mitochondrial fission and mtDAMPs may result in new therapeutic approaches to CS-induced vascular injury in the lung and potentially also in the systemic circulation.